The present invention relates to the recovery of volatile organic compounds (VOC) and, more particularly, to the recovery of volatile organic compounds associated with the fermentation processes, and, still more particularly, to the recovery of ethanol produced as a consequence of yeast-based fermentation processes associated with the production of wines, beers, brandies, rum, distilled spirits, etc. in which carbon dioxide (CO2) is co-evolved with ethanol (EtOH).
As an example, during the production of wine, a mixture of crushed grapes, juice, and yeast is placed in a fermentation vessel; the yeast metabolizes sugars in the grape juice (known as “must”) over a period of several days to one or two weeks at a process temperature of about 60-90° F. During this fermentation period in which the liquid component of the must evolves into the new wine, both EtOH and CO2 evolve in equal molar amounts as a consequence of yeast metabolism. Governed by the chemical properties of solubility, vapor pressures in the headspace, and other chemical and physical properties, the ethanol and the carbon dioxide enter the headspace within the fermentation vessel above the surface of the must. In typical wine-making processes, the ethanol vapor and the CO2 gas escape from the fermentation vessel into the ambient atmosphere during normal venting or when the fermented liquid is subject to inspection/testing and at other points in the production process.
The volume of CO2 evolved during fermentation is a function of the product of the CO2 gas volume per mole, the number of moles of CO2 per liter of liquid, and a temperature term, i.e.,: Volume(CO2)=[Volume(CO2)/mole]*[moles(CO2)/Liter]*[Ftemp] as presented by Roger B. Boulton, et al., Principles and Practices of Winemaking (New York, Springer Science+Business Media, Inc., 1999); assuming a fermentation temperature T of 68° F. (20° C.) and molar and weight values as presented in the following representative equation, about 56 Liters of CO2 per Liter of liquid is evolved (assuming a 24 Brix reduction):
                    =                                            22.4              ⁢                                                          ⁢              Liter                                      mole              ⁡                              (                                  CO                  ⁢                                                                          ⁢                  2                                )                                              *                                    210              ⁢                                                          ⁢              grams                        Liter                    *                                    1              ⁢                                                          ⁢              mole              ⁢                                                          ⁢              sugarrams                                      180              ⁢                                                          ⁢              grams                                *                                    2              ⁢                                                          ⁢              moles              ⁢                                                          ⁢              CO              ⁢                                                          ⁢              2                                      1              ⁢                                                          ⁢              mole              ⁢                                                          ⁢              sugar                                *                                    273.2              +                              T                ⁡                                  (                                      °                    ⁢                                                                                  ⁢                                          C                      .                                                        )                                                      273.2                                              EQ        ⁢                                  ⁢        1            
FIG. 1 is a graphical representation of the Boulton equation for must in a 50,000-600,000 gallon range and shows potential CO2 emitted in the 400,000 to 4.5 million cubic feet range with a linear slope and is based on the Boulton equation with fermentation at 30° C. (86° F.) and a 23° Brix reduction (or 201.3 grams/Liter).
The EtOH emission factor EF (lbs ethanol lost/1,000 gal of wine made) is given by the formula:EF=(0.135T−5.91)+(B−20.4)(T−15.21)(0.0065)+C  EQ 2                where:                    T=fermentation temperature ° F.            B=initial sugar content, ° Brix. (typical full fermentation reduction is about 20.4° B.)            C=0 for white wine and 2.4 lb/1,000 gal for red wine                        
The EtOH emission due to temperature and change in the Brix is given by the formula:log(Elost[(So−S]2)=K4−K5/(T+273)  EQ 3                where:                    Elost=ethanol emitted (g/L)            So=initial sugar concentration (g/L) S=final sugar concentration (g/L)            T=fermentation temperature (° C.)            K4, K5=constants, 6.682 and 2,552 respectively                        
Of the two emission formulas above, the first (EQ 2) is a representation of the United States Environmental Protection Agency (EPA) formula for red and white wine EtOH emission at a specified starting Brix with the temperature variable, and the second formula (EQ 3) is a representation of the Williams-Boulton formula for EtOH emission from white wine at a specified starting and finished Brix with a temperature variable.
FIG. 2 is a graphical representation of the potential EtOH emission factor EF for must in a 50,000-600,000 gallon range and shows emitted EtOH in the 400 to 5000 lb range for red wine with a linear slope and emitted EtOH in the 200 to 2200 lb range for white wine, also with a linear slope. FIG. 2 is derived from EQ 2 with a 23° Brix reduction.
FIG. 3 is a graphical representation of the potential EtOH emission factor EF for a 100,000 gallon must as a function of temperature showing a range of about 160-350 lbs for white wine in a 54-70° F. range and about 610 to 970 lbs for red wine in a 70-96° F. range. FIG. 3 is also derived from EQ 2.
The loss of gaseous EtOH into the ambient atmosphere is undesirable since EtOH (as well as other VOC emissions from a variety of other industrial, mobile, and natural sources) in the presence of oxides of nitrogen, reacts in sunlight to produce ozone. This has led to regulations in certain wine-producing areas of California to encourage wineries to reduce their EtOH emissions.
A number of attempts have been made to recover the ethanol vapors from the wine-making process and have been recognized as not feasible because they have the potential to jeopardize the quality of the wine produced or are otherwise incompatible with the wine-making process. It is generally recognized that any recovery process must not or may only minimally impact the fermentation process to insure product quality. Thus, recovery systems that change or impact the equilibrium of the fermentation-created conditions in the headspace (temperature, pressures, constituent ratios, etc.) are unacceptable because of the risk to the quality of the final product.
To date, active control systems utilizing thermal oxidation, catalytic thermal oxidation, regenerative thermal oxidation, wet scrubbing (absorption), adsorption vapor recovery, and condensation, refrigeration, and cryogenic systems have been considered untenable in the wine-making system. In an Apr. 30, 2007 report, the San Joaquin Valley Unified Air Pollution Control District, which has adopted the first winery-specific ethanol regulations in the world, has stated, “Currently there is no achieved in practice control technology to control VOC emissions from wine fermentation or brandy aging.” and “there is concern that emissions control could contaminate the product or impact wine quality consistency.” Based on these concerns, the District “believes that there is no feasible RACT-level control for wine fermentation, wine storage tanks, and brandy aging.” Further, a report from the San Joaquin Valley Unified Air Pollution Control District found that the US Environmental Protection Agency's emission control database “contains no examples of controlling wine fermentation emissions.”
Traditional methods of emission control have not proven feasible as they risk interference with the natural fermentation process. Of utmost importance for the process is to maintain a friendly environment for the yeast and ensure the integrity of the finished wine product. Any change in the headspace conditions runs the risk of a sub-optimal product and the consequent economic loss in the marketplace.